Use of the throttle as the primary ground speed control of a mobile earth working machine, such as a wheel loader, is generally precluded by the need to maintain full engine power to the auxiliary hydraulic equipment at low speeds. For example, if the throttle were reduced to slow the machine as the bucket enters a pile of material, there might be insufficient power available to lift the bucket though the pile for loading. Accordingly, ground speed has conventionally been controlled by braking and operation of a "left pedal", which mechanically disengages a clutch between the engine and transmission.
Manually controlling the degree of clutch slip needed to achieve a desired ground speed can be tiring for the operator. Further complicating the task is the need for the operator to simultaneously steer the machine and operate the hydraulic implement controls. It is therefore not surprising that various forms of assisted ground speed control have been found to contribute significantly to overall productivity and operator comfort.
It is further desirable to assist the operator with full throttle directional shifts, whereby the direction of rotation of the transmission output is reversed while the engine remains at close to full throttle. As the new gear and impeller clutch are fully engaged, uneven deceleration/acceleration often results in unacceptable levels of jerk, causing machine wear and operator discomfort, which can be reduced to some extent by first manually braking the machine.
In U.S. Pat. No. 5,040,648 to Mitchel et al., the impeller of a hydrodynamic torque converter is selectively engaged to the engine by hydraulically actuated impeller clutch discs, in order to adjust the ground speed at full throttle. An electrohydraulic system supplies pressurized fluid to the impeller clutch, for controlling the torque transmitted to the drive train in proportion to an impeller pedal position. Depression of the impeller pedal beyond a predetermined point may also progressively supply pressurized fluid to the service brakes for further reductions in ground speed. It remains difficult however, to maintain a controlled ground speed using such open loop control due to changes in torque on the drive train caused by obstacles, wheel slip and uneven terrain.
U.S. Pat. No. 5,509,520 to Evans et al. discloses a controller which utilizes proportional-integral (PI) closed loop control of an impeller clutch and brakes to maintain a desired ground speed. A reference ground speed is stored at the moment the impeller pedal is depressed and used to calculate a desired speed as a function of the impeller pedal position. An error signal based upon a comparison of desired speed with the current ground speed is fed back to the controller for generation of impeller clutch and brake pressure command signals to reduce the speed error.
PI control is very sensitive to modeling and measurement errors, and can not guarantee stability with varying drive train dynamics. While the aforementioned PI control is adequate for steady state conditions, it is slow to respond to transient conditions due to the low gain levels selected to avoid overshoot and chatter (caused by rapid switching between the brake and impeller clutch). Furthermore, system delays or dead time, such as fill times for the hydraulic actuator valves, may cause excessive wind-up in the integrator and result in unacceptable jerk as the valve engages abruptly.
The present invention is directed to overcoming one or more of the problems as set forth above.